PA140st arc stability

[joshuab]

The problem seems to be in the lift-start circuitry. I have switched to stick mode and sacrificed the electrode a bit during arc-start, but the arc is stable and it welds OK.

You know, I didn't realize you were talking about the TIG torch when you said it welds fine in stick mode. You may already know this, but what you're describing is known as scratch-start TIG, and it's been a standard way of running a TIG torch off of a stick welder for years. HF is required for AC TIG, and as you point out, lift start or HF will help preserve your electrodes, but scratch start is tried and true.

One interesting point: Most of the comparison tests I've seen are all being run at higher currents than what I need. Is it a lot trickier to use these machines at low currents?

I have a PA160-STH. Not the exact same machine as yours, but not a high-end purpose-built TIG welder either. It's rock solid at whatever output I set it to, high or low. None of this wavering that you talk about. I don't typically weld at low amperage, and I use HF start when I do, so I can't speak to how its lift start works at low amperage.

It's so weird that you have two machines with exactly the same problem. It sounds like you're no stranger to welding, and like you have ruled out many of the obvious things except for the machine itself.

[Rambozo]

OK, I checked again and here is what I found. Open circuit voltage is right at 69 volts and is bang on over the whole amp range. There is some thermal drift over time, but it's less than .1 volt and again is constant over the whole amp range. This is the same in stick mode and TIG mode, BTW.



So clearly you have a problem with your machine and how far you want to dive into it is up to you. Very odd that you got two bad ones.

Mine is very stable down to 10 amps on 240VAC sorry I forgot to check on 120VAC, but as I recall it will go down to 7 or 8 on 120VAC. The actual output is a little higher than what is displayed, and that is pretty consistent everywhere. Checking under load, the output is very stable with just a little droop at longer arc lengths. In the low range it can handle arc lengths up to about 15 volts before it cuts out. With the panel set to 10 amps, a short arc would yield 13.6 amps @ 11.5 volts, while a super long arc would be 12.2 amps @ 14.8 volts. At the 15 amp setting the results are about the same 18.9A @ 11.5V vs 18.0A @ 13.3V



You answered my one other question about checking in stick mode. I frequently forget to flip the switch, and not only will the automatic hot start take it's toll on the tungsten, but the auto arc force also plays with the amperage and causes some of what you describe. I wonder if that is not being switched out on your machine? As the voltage goes down it will crank up the current but it is not reflected in the panel display reading.


BTW joshuab, using this kind of machine is stick mode is not quite the same as a normal scratch start setup on a true CC machine. There is that annoying auto hot start and auto arc force that can't be turned off. Also HF is not required on all AC TIG. True sinewave needs HF on all the time to stabilize the arc through the zero crossings, especially at low power. Squarewave machines can get by with just HF start or none at all since they have very rapid zero crossings.

[NWD43]

OK, I checked again and here is what I found. Open circuit voltage is right at 69 volts and is bang on over the whole amp range. There is some thermal drift over time, but it's less than .1 volt and again is constant over the whole amp range. This is the same in stick mode and TIG mode, BTW.



So clearly you have a problem with your machine and how far you want to dive into it is up to you. Very odd that you got two bad ones.

Mine is very stable down to 10 amps on 240VAC sorry I forgot to check on 120VAC, but as I recall it will go down to 7 or 8 on 120VAC. The actual output is a little higher than what is displayed, and that is pretty consistent everywhere. Checking under load, the output is very stable with just a little droop at longer arc lengths. In the low range it can handle arc lengths up to about 15 volts before it cuts out. With the panel set to 10 amps, a short arc would yield 13.6 amps @ 11.5 volts, while a super long arc would be 12.2 amps @ 14.8 volts. At the 15 amp setting the results are about the same 18.9A @ 11.5V vs 18.0A @ 13.3V



You answered my one other question about checking in stick mode. I frequently forget to flip the switch, and not only will the automatic hot start take it's toll on the tungsten, but the auto arc force also plays with the amperage and causes some of what you describe. I wonder if that is not being switched out on your machine? As the voltage goes down it will crank up the current but it is not reflected in the panel display reading.


BTW joshuab, using this kind of machine is stick mode is not quite the same as a normal scratch start setup on a true CC machine. There is that annoying auto hot start and auto arc force that can't be turned off. Also HF is not required on all AC TIG. True sinewave needs HF on all the time to stabilize the arc through the zero crossings, especially at low power. Squarewave machines can get by with just HF start or none at all since they have very rapid zero crossings.

Thanks for taking the time do do the testing! I have inserted a high-wattage shunt (1 ohm) in series with the ground lead and measured the current under various conditions:

Dial set to minimum (11A), with electrode touching work the current is 8.17A
Dial set to 30, current measures 20.5A
Dial set to 50, current measures 27A

With dial set to 20 (the lowest setting that will establish an arc, current measures 9.7 to 10.1A
In order to get 20 A of measured current, dial must be set to 40A

Here's the interesting part- with the 1 ohm shunt in place, the variations in current and the flashing duty cycle lamp go away!

Gotta think about this while waiting for CA to wake up....... Neil

[performance]

Internally Rambozo, that is a much different machine that you have than the newer ones with the rocker switch...though performance should not be vastly different.

[performance]

It sounds as if the display has some calibration issues.

[NWD43]

It sounds as if the display has some calibration issues.

According to the phone tech. if the duty cycle/OT lamp lights it indicates an internal problem with the machine. Have another one on the way for the third and final test. A product review of the PA140 on Amazon related the exact same problem but the reviewer was so inexperienced that he thought it was just a normal characteristic of the machine. I'm learning things I probably should already know about price/performance expectations.

With crossed fingers, Neil

[performance]

We've had very very few issues out of the 140ST...or any of the powerarc welders.

[NWD43]

We've had very very few issues out of the 140ST...or any of the powerarc welders.

I believe what you say because one of the things that enticed me to buy this model was the lack of much negative stuff on the forums.

I got the new one today (Amazon is AMAZING as far as returns go) and fired it up as fast as I could. This one seems to work much better than the other two. I tried a bunch of stuff at around 20 Amps and it works very well. Unlike the others, I can strike an arc at an indicated 10 Amps, but the duty cycle lamp flashes and the arc goes out. As soon as I increase the current to about 12 on the display it works OK, so it's an improvement but still not "as advertized" performance. I tried some 22 gauge stainless and cannot reduce the current enough to do nice work, big disappointment since that's typically the kind of thing I do. Gotta keep remembering I can't compare it to the Lincoln.....

I have heard several times from tech support that the duty cycle lamp should NOT light at any time other than to indicate over-temperature. If it does then something is wrong. Since the lamp always lights during lift-arc starts, I assumed that was normal (book does not mention this) but should never light otherwise. Since the lamp flashes during low-current operation, I'm sure that the problem is with the lift-arc circuitry.

I'm kind of sick of yappin' about this. Just wish the units would work as they should, but every time I get too picky I remember the price, and being a veteran of the product development business I know that when the money comes out something is traded off. I'll probably keep this one since it does most of what I want and I need the portability.

Thanks to all for the generous help and advice, Neil

[Rambozo]

I believe what you say because one of the things that enticed me to buy this model was the lack of much negative stuff on the forums.

I got the new one today (Amazon is AMAZING as far as returns go) and fired it up as fast as I could. This one seems to work much better than the other two. I tried a bunch of stuff at around 20 Amps and it works very well. Unlike the others, I can strike an arc at an indicated 10 Amps, but the duty cycle lamp flashes and the arc goes out. As soon as I increase the current to about 12 on the display it works OK, so it's an improvement but still not "as advertized" performance. I tried some 22 gauge stainless and cannot reduce the current enough to do nice work, big disappointment since that's typically the kind of thing I do. Gotta keep remembering I can't compare it to the Lincoln.....

I have heard several times from tech support that the duty cycle lamp should NOT light at any time other than to indicate over-temperature. If it does then something is wrong. Since the lamp always lights during lift-arc starts, I assumed that was normal (book does not mention this) but should never light otherwise. Since the lamp flashes during low-current operation, I'm sure that the problem is with the lift-arc circuitry.

I'm kind of sick of yappin' about this. Just wish the units would work as they should, but every time I get too picky I remember the price, and being a veteran of the product development business I know that when the money comes out something is traded off. I'll probably keep this one since it does most of what I want and I need the portability.

Thanks to all for the generous help and advice, Neil

You could compare it to a Lincoln, but only to something like an Invertech V155-S

No one has ever mentioned the duty cycle lamp flashing, I do not think that is normal at all. Out of curiosity, have you tried it on 120 volts? I find that for real low amp work, it's much more controllable when running on low voltage. I have added a finger amptrol, and always light up at 10 amps and its very stable. I am wondering just what they changed in the new version. I wasn't expecting it to be a real low amp TIG unit as that is not what it was designed for, but I have been more than pleased with it's performance. On 120v the displayed current was 8 - 9 amps to do this with some .030 MIG wire for filler. Not a great job, but that's more my vision than the machine.

[joshuab]

BTW joshuab, using this kind of machine is stick mode is not quite the same as a normal scratch start setup on a true CC machine. There is that annoying auto hot start and auto arc force that can't be turned off. Also HF is not required on all AC TIG. True sinewave needs HF on all the time to stabilize the arc through the zero crossings, especially at low power. Squarewave machines can get by with just HF start or none at all since they have very rapid zero crossings.

You're 100% right. These were details that I chose to leave out.